Image device



July 19, 1960 A. E. ANDERSON IMAGE DEVICE Filed July 18, 1957 INVENTORArr hu r E. Ander son flail 14% ATTORNEY IMAGE DEVICE Arthur E.Anderson, Penn Township,'Allegheny County, Pa., assignor to WestinghouseElectric Corporation, East Pittsburgh, Pa., a corporation ofPennsylvania Filed July 18,1957, Ser. No. 672,667

4 Claims. (Cl. 313-65) This invention relates to electron dischargedevices and more particularly to those tubes in which a radiation imageis received on an input screen and produces output signals which may beused to reconstruct the radiation image. This invention is particularlyuseful in pickup tubes such as those describedrin copending applicationSerial No. 416,879, entitled X-Ray Image Intensifying Device, filedMarch 17, 19.54, and an application entitled An Image Device, Serial No.584,231, filed May 11, 1956, and issued March 15,1960, as Patent2,928,969, both by R. J. Schneeberger and assigned to the same assigneeas the present invention.

In an image intensifying device of this type a radia: tion image such asX-rays is focussed uponan. input screen which emits electrons at a rateproportional to the brightness of each element. The radiation may befocussed directly on a photoemissive material or may be converted bymeans of a phosphor into another wave length of radiation to which thephotoemissive material is sensitive. The electrons within thephotoelectron image generated by the input screen are accelerated to avelocity of the order of to :20 kilovolts and focussed to a reduced sizeupon a target electrode. The target member is comprised of asemi-insulating layer of a material exhibiting the property of electronbombardment induced conductivity with an electrical conductive backingplate on the side of the semi-insulating layer racing the input screenand on which the photoelectron image is focussed. The photoelectronsfrom the input screen penetrate through the thin electron permeableelectrical conductive backing layer of the target and penetrate into thesemi-insulating layer. The semi-insulating layer has a resistivity inthe order of 10 ohms per cubic centimeter. The penetration of theelectrons into the semiinsulating layer produces what may be thought ofas a conductive image duplicating the spaced distribution of theelectron image from the input screen and so of the radiation imagedirected onto the input screen.

It is necessary that a polarizing voltage be applied across thesemi-insulating layer. The backing plate serves as one of the electrodesfor applying the field across the semi-insulating layer and instead of asecond electrode on the other surface of the semi-insulating layer anelectron gun is utilizedto deposit a charge on the exposed surface ofthe semi-insulating layer. vThe electron gun scans the exposed or freesurface of the semi-insulating layer to bring the entire surface to apotential similar to that applied to the-cathode of the of thesemi-insulating layer of thetarget facing the electron gun is maintainedat the cathode potential by the electron scanning beam. A'positivepotential is applied to the backing plate of the target of the order of50 volts positive with respect to the cathode'potential of the scanningbeam gun and therefore a field of 50 volts The semi-insulating layeracts in a manner similar to a leaky capacitor resulting in the scannedor free surface of the semi-insulating layer changing from the cathodepotential of the scanning gun to some positive potential not greaterthan the target voltage. This effect on the semi-insulating layer may bereferred to as a conductivity image. The change in potential or chargeon the WP face elements of the scanned surface of the semi-insulator isproportional tov the intensity of the electronborn bardme'ntby'thephotoelectrons from the input screen.

When the low velocity scanning electron beam 'moves across thesurface ofthetarg'eteach target element be, restoredto the catho-de;potentialofthescanning "beam gun. A signal may be-derived from the backing platecorresponding to the restoring charge derived from i the scanning beam.The output signal maybe employed in'a conventional manner fortransmission or direct con-- nection to a conventional display device. p

The device described in the above-mentioned applications obtains a verygood signal to noise ratio at low levels of radiation. energy. A targetsuch as described above using a semi-insulating material of arsenictrisulfide has been found to exhibit amplific'ations as high as onethousand with an accelerating voltage of 20 kiloe volts and a field ofvolts. It is advantageous to vobtain a signal to noise ratio as high aspossible with a given scene brightness. For example, a signal to noiseratio of 50 is a relatively snow-free picture. It is found that a targetcurrent of 10* amperes is needed to produce a signal which is 50 timesthe noise power inherent with 'a target amplification of one thousand, aphotoelectron beam generating gun. In:the absence of a radiation imagedirected onto the input screen, the surface emissivc current of 10amperes is required to produce an output current of 10*" from thetarget. A typical photoemissive cathodewill emit electrons at the rateof 1O- ampercs at the lowest fluoroscopic levels used in presentinvention. The image orthicon utiliz es amplifies;

For example,

tion of the return portion of the scanning beam in a wellknown manner.The disadvantage of this type of amplification is that the maximumreturn beam current and therefore the maximum noise occurs with minimumvideo or low light level signals. It is therefore desirable to placesome type of internal amplification within the pickup tube and prior tothe scanning beam and therefore derive, if possible, the amplifiedsignal directly from the target. This type of amplification contributesonly a negligible amount of noise to the amplified signal obtained fromthe target electrode. In particular this invention is directed to apickup tube in which the image section is modified in order to obtain amaximum amount of pho'toelectrons bombarding the target member with agiven light level.

It is therefore an object of my invention to provide an improved type ofpickup tube.

It is another object to provide an improved pickup tube having highsignal to noise ratio.

It is another object to provide a pickup tube having acceptablereproduction of a scene at extremely low brightness levels.

It is another object to provide a pickup tube which permitsself-dividing a scene of fixed brightness into smaller elements whilemaintaining the original signal to noise ratio.

It is another object to provide an improved pickup tube having a targetelectrode depending on electron bombardment induced conductivity typematerial.

These and other objects are effected by my invention as will be apparentfrom the following description taken in accordance with the accompanyingdrawing throughout which like reference characters indicate like partsand in which:

Figure l is a schematic view partly in section of a tube embodying theprinciples of my invention;

Fig. 2 is a sectional view on an enlarged scale of the input screen inFig. 1; and

Fig. 3 is a sectional view of a modified input screen.

Referring in detail to Figs. 1 and 2, a vacuum-tight enclosure 01"envelope 10, which may be of any suitable material such as glass and ofany suitable configuration, is provided. In the specific embodimentshown, the envelope is essentially a tubular member closed at one endwith an input window 12 and the other end closed with a button-like stem13. The button stem 13, normally of glass, co'nsists of a centrallylocated tubular member 14 for evacuating and sealing off the envelope 10and also a plurality of lead-in members 16 for applying voltages to theelectrode members within the envelope. An input screen 20 is normallypositioned on the input window 12 of the tube or may be in some casesseparated therefrom and provided with a transparent glass supportmember. In the specific embodiment shown, the input window 12 serves asthe support mem her for the input screen 20 and is provided with atransparent conductive coating 22 on the inner surface of the window 12of a material such as stannic oxide. An inlead connected to thetransparent electrical conductive coating 22 is provided to the exteriorportion of the envelope 10. The Window 12 and coating 22 are transparentto the wave lengths of the radiations to be detected. A light-sensitivelayer 24 of a photoconductive material is positioned on the exposedsurface of the transparent conductive layer 22. The layer 24 is of amaterial that exhibits the property of a decrease in resistivity whenexposed to radiations to be detected, such as X-rays, visible light andinfrared. It is necessary to select the specific photoconductivematerial in order that it be responsive to the Wave length of theradiation. Suitable photoconductive materials are selenium, antimonytrisulfide,-.c admium sulfide, lead telluride or any other knownphotoconductive material.

Deposited on the surface of the photoconductive layer 24 is anelectron-emissive layer 26. The electron-emis- 4. sive layer 26 is of amaterial that emits electrons in response to radiation such as gold,cesium antimonide, cesium-silver-dxygen or any one of the many otherwellknown photoemissive materials. The layer 26 may be continuous ormosaic. In the structure shown in Figs. 1 and 2, a mosaic layer 26 isshown. The layer 26 is illuminated by a light source 28. The wave lengthof the light source 28 is such as to be in the region which the materialin the layer 26 is sensitive. In this embodiment, it would also benecessary that the photoconductive layer 24 be insensitive to radiationsfrom the auxiliary light source 28 and the electron-emissive layer 26must be insensitive to the input radiations.

An alternative input screen is shown in Fig. 3. The screen consists ofan interleaved structure of opaque conductive elements 32 and insulatingelements 34 evaporated over the photoconductive layer 24. Thephotoemissive elements 36'would then be deposited over the conductiveareas. The conductive elements 32. can be any of a number of metalscompatible with the particular photoconductive materials andphotoemissive materials utilized. The insulating'material'in theelements 34 must also be compatiblewith the other materials and beopaque to any radiation from auxiliary source 28 to which thephotoconductive layer 24 is sensitive. It is also obvious that theinsulating material used should not be photoconductive to the auxiliaryradiation. In the structure shown in Fig. 3, gold could be used for boththe conductive elements 32 and the emissive elements 36 and antimonytrisulfide as the insulator ele-j ments 34 in the case where theradiation from the auxile iary source 28 was of the wave length of about2,537 angstro'm units in the ultraviolet region. Both the gold andantimony trisulfide should be several microns thick in order to preventtransmissionv of the ultraviolet light from the auxiliary light5011112623 onto the photoconductive layer 24. I v v A grid member 49 ispositioned adjacent to the. input screen 20 and is of a suitableconductive material such as nickel having large open areas. j An inlead23 isalso provided from the grid electrode 40 to the, exterior per tionof the envelope.

A target member Si is positioned on theopposite side of the grid member40 with respect to the input screen 20 and is comprised essentially ofthe metallic support mesh structure 52 of a material such as copper, ornickel having a large open area. On thevopposite side of the supportmesh 52 with respect to the input screen is a continuous backing layer54 of a suitable electrical conductive material such as aluminum. Thebacking layer 54 is sufficiently thin so as to be essentially permeableto electrons accelerated from the input screen 20. On the oppositesurface of the backing layer 54 with respect to the mesh supportstructure 52 is deposited a thin layer 56 of a semi-insulating materialof a resistivity greater than 10 ohms per cubic centimeter in theunexcited state such as arsenic trisulfide, antimony trisulfide, oramorphous selenium which exhibit the property of elec-. tron bombardmentinduced conductivity. The backing layer 54 is also provided with aninlead 58 to the exterior portion of the envelope 10.

The portion of the tube and electrodes on the input screen side of thetarget 50 is normally referred to. as the image section of the pickuptube. It is necessary to provide electrostatic or magnetic means forfocussing the electrons emitted from the input screen 26 and also toprovide acceleration to the target member. In the specific embodimentshown, a focussing coil .59 is illustrated for providing the necessaryfocussing of the ,electrons emitted from the input screen 20 anddirected onto the target member 56. The acceleration is provided bymeansofasuitable voltage source 7 4 connected between the inleads 23 and58. The section of the tube on the opposite side of thetarget member 50withrespect' to theinput screen 20 is normally referred to as thescanning an ers E ea Section of the pickup tube; An electron gun 60 forgenerating an electron beam of low velocityis provided within thescanning section of the'tube, and the electron beam is directed onto theexposed surface ofthesemiinsulating layer 56 of the target member 50:The elecof the target 50 by means of 'deflecftion'coils, focussing.

coils and alignment coils of suitable forms well known in the art whichare illustrated by coils 61,- 63-and 65. Inleads from the cathode 62,control grid 64, accelerating electrode 66, and anode 68 pass throughthe button stem 13 of the tube, and suitable potentials-are appliedthereto. In the specific embodiment shown in Fig. 1, the cathode 62 isshown connected to 'ground'for the purposes of explaining the operationofthe device.

The inlead 25 is connected to the source 74 so-that the input screen 26normally operates at a'potential of the order of 20,000 kilovoltsnegative with respect to ground, and the control grid 40 operates at apotential of the order of 50 to 100 volts positive with respect to theinput screen '20 by means of a battery 27. The inlead 58 to the targetelectrode 50 is connected through an output resistor 7 0 to a voltagesource 29 to provide an operating voltage of about 50 volts positivewith respect to ground, and the voltage developed across the outputresistor 70 is connetced to a suitable video amplifier 72.

The mode of operation of the device may be described somewhat in thefollowing manner. If it is assumed that the radiation image is notprojected onto the tube, the scanning electron gun 65 which is'operatingbelow first crossover of the semi-insulating material layer 56,

will charge the surface to the potential of the cathode 62' of the gun6i) which is ground potential. The backing layer 54 will be 50 voltspositive with respect toground and the scanned surface of thesemi-insulating layer-56$ The potential difference between the inputscreen and the target is of the order of to kilovolt's. It is necessaryto operate at such potentials in order to induce conductivity within thetarget member 50. The conductivity induced in a given material underelectron bombardment is a functionof both the thickness of the materialand the intensity; that is, energy or numbers of the bombardingelectrons. The maximum gain is obtained when the bombarding electronshave just enough energy to penetrate through the backing layer 54.

Practical considerations such as variation in velocity of the readingelectrons from the electron scanning beam gun place a lower limit on theminimum voltage diiierence which can be read out. As a result, thedetectable charge difference becomes smaller as the capacitance perelement decreases or as the film thickness increases. In practice, asemi-insulating film thickness of the order of 1 to 2 microns andbombarding energy of 10 to 20 kilovolts appears to be of optimum value.The voltage across the photoconductive layer 24 must be limited to 50 to100 volts or the layer will be destroyed. It is therefore necessary toinsert the grid member 40 between the input screen 20 and the target 50in order to prevent voltage breakdown in the photoconductive layer 24 inthe input screen 20. In operation, this grid is made 50 to 100 voltspositive with respect to the conductive layer 22 in the input screen.

If it is now assumed that the auxiliary light source 28 continuouslyilluminates the electron-emissive layer 26 of the input screen 26 withno input radiation image directed on the photoconductive layer 24, thephotoconduc; tive layer '24 is of high resistivity, and the surfaceofthe electron-emissive layer 26 will tend to. char e positively towardthe potential of the grid 40., v This potential may be referred'to asthe equilibrium "potential; It is importantthatthe grid'40 be spacedvery closely to the elect-ron-emissive layer 26, and normally itsspacingshould be equivalent to the spacing between the adjacent wires ofthe mesh member 40. Inthis state, .very fe electrons will be acceleratedto the target 50. r

If it now is assumed that the radiation image isdirected ontothe inputscreen 20, the radiation image will cause thephotoconductive layer 24 tobemore conducrive corresponding to the radiation incident on eachelemental area of layer 24.- The corresponding elemental areas on thesurface ofjthe electron emissive. layer 26 will tend to go negativewithrespect to the equilibrium potential which will allow the electronsemitted from the electron-emissive layer 26, due to radiations fromsource 28, to pass through the grid member 44} and be accelerated to thetarget electrode 50.' The number of electrons emitted by theelectron-emissive layer 26 willsubstantially be, equal to the chargepassed through'the'photocon ductive layer 24 which would correspond tothe change in conductivity of the ph'otoconductive layer 24 in responseto the intensity of the radiation.

The electrons thus emitted from the input screen 20 would form anelectron image which duplicates the space distribution and theintensity'of the radiation image projected onto the input screen 20. Theelectron image'thus generated from the input screen 20 is accelerateda'ndfo cussed by means of the potential applied between the input screen20 and the target member 50 and also by the magnetic field provided bythe coil 59. The electron image from the input screen 20 produces aconductive image inthe semi-insulating layer 56, I and the respectiveareas of the exposed 'surfaceof the semi-insulating layer rise fromground potential to a fraction of-the potential applied; to theconductive backing layer. 54 of thetarget.

This potentialpattern on the'surface corresponds to the 1 variation ofthe conductivity imageoverfits surface; When the electron beam'generatedby the electron gun 60" Strikes a particular element or area on thesurface of thesemi-insulating layer 56 it depositssuflicient electronsto recharge the potential on the surface of the semi-insulater to thepotential of cathode 62 of the-scanning beam gunl6t). A low velocitybeam is utilized'in this structure; As a result of this rechargingaction of the scanning beam,

a corresponding charge current flows through the output resistor 74connected to the target 50 and may be used in a well-known manner. Theoutput signal may be used to modulate a television transmitter or may beconnected radiation image directed onto the tube, the firstpulse oflight from the auxiliary light source 28 would cause theelectron-emissive layer 26 to charge to a potential substantially equalto the potential on the grid 46 in what may be again referred to as anequilibrium potential. With the auxiliary light source 28 turned off andthe radiation image directed onto the input screen 20, theelectron-emissive layer 26 would be charged negatively with respect'tothe equilibrium potential so that a charge pattern would be built uponthe surface of the electronemissive layer26 corresponding to theradiation image directed on the photoconductor layer 24. In thisoperation, the resistivity of the photoconductive layer 24 is decreased,and the charge is allowed to flow through the photoconductive layer 24to the electron-erhissive layer 26.; When the auxiliary light source 28is again pulsed, the electronsfrom the input screen 20 from elementalareas having a more negative potential than the equielectrons allowed topass through the grid 40. will substantially correspond to the amount ofvoltage diiferc nlcc between the elemental areas and the equilibriumpotential of the 'electron-emissive layer 26. 1

By utilizing the photoconductive layer 24 in combination with theele'ctron-emissive layer 26, one is able to obtain electron multiplyingproperties of thin insulating films combined with the high quantum yieldof photoconductors in order to obtain a high sensitive radiation imagepickup tube. Photoconductive materials have quantum yields much greaterthan one, while photo emissive materials have yields of only about 0.1.

While I have shown my invention in several forms, it will be obvious tothose skilled in the art that it is not so limited but is susceptible tovarious other changes and modifications without departing from thespirit and scope thereof.

I claim as my invention:

1. An electron discharge device comprising a target member including afilm of material exhibiting the property of electron bombardment inducedconductivity above a predetermined value of electron energy, means forgenerating an electron beam having energies below said predeterminedvalue to bombard one surface of said film to establish an equilibriumpotential on the surface of said film, an electrical conductive sheet incontact with the opposite surface of said film to maintain the oppositesurface at a fixed potential different from said equilibrium potential,an input screen spaced from said target for projecting electrons havingenergies above said predetermined value onto said target member to setup a conductivity image within said film corresponding to the electronimage generated by said input screen, said input screen comprising alayer of photoconductive material sensitive to the radiation of theimage to be detected sandwiched between a photoemissive layerinsensitive to said input radiation and a conductive layer, saidphotoemissive layer facing said target, a grid member positioned betweensaid photoemissive layer and said target member and adjacent to thesurface of said photoemissive layer, means for applying a potentialbetween said conductive layer and said grid and means for illuminatingsaid photoemissive layer with radiation to which said photoemissivelayer is sensitive and to which said photoconductive layer isinsensitive.

2. An image pickup tube comprising a vacuum-tight envelope having aphotosensitive input screen adjacent one end of said envelope, a targetmember positioned from said input screen within said envelope, saidtarget comprising a thin conductive layer and a layer of semiinsulatingmaterial supported on the surface of said conductive layer opposite fromsaid input screen, said layer of semi-insulating material exhibiting theproperty of electron bombardment induced conductivity upon impact ofelectrons thereon, said input screen comprising a layer of electricalconductive material transparent to radiations of a first wave lengthregion in which said image pickup tube is utilized, a photoconductivelayer positioned on said conductive layer on the side thereof facingsaid target, said photoconductive layer being sensitive to radiationswithin said first wave length region, a mosaic conductive layerdeposited on the exposed surface of said photoconductive layer andopaque to radiations within said first wave length region, aphotoemissive layer disposed on said conductive mosaic layer andsensitive to radiations within a second wave length region, aninsulating material deposited within the openings of said mosaicconductive and photoemissive layers and opaque to radiations within saidfirst and second wave length regions, a grid member positioned adjacentto said photoemissive layer and between said photoemissive layer andsaid target, means for illuminating the photoemissive layer with anauxiliary light source of radiations within said second wave lengthregion, means for scanning the surface of said semi-insulating layer ofsaid target with an electron beam to derive a signal from saidconductive layer of said target in response to the removal of the chargepattern due to the conductivity pattern induced in said semi-insulatinglayer in response tq bornbardment. of said target member with electronsfrom saidinput screen; r 1 t l I."

3. An image pickup tube comprising a vacuum-tight envelope having aphotosensitive input screen adjacent one end of said envelope, a targetmember positioned from saidlinput screen within said envelope, saidtarget comprising a thin conductive layer andalayer of semiinsulatingmaterialsupported on the surface of said conductive layer opposite fromsaid input screen, said layer of semi-insulating material exhibiting theproperty of elec: tronbombardment induced conductivity upon impact ofelectrons thereon, said input screen comprising a layer of electricalconductive material transparent to radiation within a first wave lengthband in which said image pick; up tube is utilized, a photoconductivelayer positioned 'on said conductive layer on the side thereof facingsaid target, said phtoconductive layer being sensitive to radia: tionwithin said first wave length band, a mosaic con ductive layer depositedon the exposed surface of said photoconductive layer and opaque toradiation within said first wave length band, a mosaic photoemissivelayer disposed on said conductive mosaic layer and sensitive toradiations within a second wave length band, an insu lating materialdeposited within the openings of said mo saic conductive andphotoemissive layers and opaque to radiation within said first andsecond wave length re'-. gions, an electrical conductive electrodehaving a plu rality of apertures positioned adjacent to said mosaicphotoemissive layer and between said mosaic photoemissive layer and saidtarget, said electrode spaced at a dis tance from saidmosaicphotoemissive layer such that the distance is subtsantially the same asthe diameter of said apertures, means for illuminating said mosaic photoemissive layer with an auxiliary light source of radiation within saidsecond wave length band, said first and sec: ond wave length bands atleast partially overlapping, means for scanning the surface of saidsemi-insulating layer of said target with an electron beam to derive asignal from said conductive layer of said target in response to theremoval of the charge pattern due to the conductivity pattern induced insaid semi-insulating layer in response to bombardment of said targetmember with elec trons from said input screen. i

4. An electron discharge device comprising a target member including afilm of material exhibiting the prop erty of electron bombardmentinduced conductivity above a predetermined value of electron energy,means for generating an electron beam having an average energy belowsaid predetermined value to bombard one surface of said film toestablish an equilibrium potential on the surface of said film, anelectrically conductive sheet in contact with the opposite surface ofsaid film to maintain the opposite surface at a fixed potentialdifferent from said equilibrium potential, an input screen spaced fromsaid target for projecting an image comprising electrons having energiesabove said predetermined value onto said target member to set up aconductivity image within said film corresponding to the electron imagegenerated by said input screen, said input screen comprising a layer ofelectrically conductive material transparent to radiation of a firstwave length band in which said image pickup tube is utilized, aphotoconductive layer positioned on said conductive layer on the sidethereof facing said target, said photoconductive layer being sensitiveto radiation within said first wave length band, a mosaic conductivelayer of a metal deposited on the exposed surface of saidphotoconductive layer and opaque to radiations within said first wavelength band, a mosaic photoemissive layer disposed on said conductivemosaic layer and sensitive to radiation within a second wave lengthband, an insulating material deposited within the openings of saidmosaic conductive and photoemissive layers and opaque to radiationwithin said first and second wave length regions, a grid memberpositioned adjacent to said mosaic photoemissive layer and said target,means for uniformly illuminating said mosaic photoemissive layer with anauxiliary light source of radiation within said second wave lengthregion, means for scanning the surface of said semi-insulating layer ofsaid target with an 5 electron beam to derive a signal from saidconductive layer of said target in response to the removal of the chargepattern due to the conductivity pattern induced in said semi-insulatinglayer in response to bombardment of said target member with electronsfrom said input 0 screen.

References Cited in the file of this patent UNITED STATES PATENTS WeimerOct. 6, 1953 Edwards July 13, 1954 Sheldon Mar. 20, 1956

